The present invention relates to an apparatus for applying a plurality of layers, which are located one on top of the other and at least one of which is magnetizable, to a moving flexible substrate by means of a plurality of coater slots arranged in a block and fed from reservoirs, and magnetic recording media thus produced.
For the production of recording media which nowadays consist in general of a plurality of layers located one on top of the other, there is a large number of different coating processes which differ very greatly depending on the structure of the recording medium and on the part-layers required for producing such recording media. Thus, photographic recording media which may consist of more than ten layers are for the most part produced by means of cascade coaters or otherwise curtain coaters. In the case of a cascade coater, which is shown schematically in FIG. 1, a plurality of dispersions flowing one above the other run simultaneously over a downward-sloping surface to the run-off edge and are taken up by a substrate moving past vertically upward, the fluid bridge formed from the layers being stabilized by slightly reduced pressure below the run-off edge. In the case of a curtain coater, the individual dispersions extruded from a coater orifice fall individually in a curtain onto the substrate moving horizontally past and are thus applied one on top of the other.
There is a plurality of methods for the application of magnetizable layers whose dispersions generally have a substantially higher viscosity. A dispersion containing the finely divided magnetic pigments, which are finely distributed in a polymeric binder matrix, can be applied to the substrate by means of knife coaters, doctor blade coaters, gravure coaters, reverse roll coaters or extruder coaters. An extruder coater for simultaneous application of a plurality of layers located one on top of the other is disclosed in PCT application EP 93/02017. The dispersions are applied to a flexible substrate from at least two coater slots, the upper edge of the upper outlet orifice arranged in the direction of transport of the substrate being recessed with respect to the lower edge of the lower outlet orifice, and a right parallelepiped magnet whose upper edge is opposite the common outlet orifice of the coater slots and from 0.1 to 5 mm away being arranged behind the substrate and parallel thereto, and a stripper plate which is extended from the extruder edge toward the material web being mounted at the two coating edges in order to avoid thickening of the edges. FIG. 2 schematically shows the structure of such a coater, which in particular has the advantage that the applied amounts of dispersion are substantially independent of the distance of the outlet slots from the material web. Magnetic recording media produced using such coaters are disclosed in DE-A 42 34 608 and 42 43 846.